In 1965 he was conscripted into the Australian Army, In 1967 he deployed to Vietnam as a Plant Operator with the Royal Australian Engineers and afterwards remained in the Army serving in Singapore and the United States until discharging as a Major in 1986. He qualified as a HALO (High Altitude Low Opening) military parachutist with the US Army in 1978. He is a graduate of the Officer Cadet School, Portsea, Victoria and the Army Staff College at Fort Queenscliff, Victoria. He represented the Army in Australian Rules football, tennis, squash and marathon running.

Following his discharge, became a Special Events Organiser. He has organised the Sydney to Melbourne Ultra Marathon, the Anzac Day Marathon, the 18,000 km (11,000 mi) Round Australia Relay for the Australian Cancer Foundation, the Great Australian Caravan Safari, the Darwin-Cairns-Melbourne Relay for the Melbourne Olympic Committee, and the international George Street Mile footrace. He was a Consultant to Australian Rural Leadership Foundation and also a Facilitator for Adventure West Leadership and Survival Training activities.

Lynn was elected to the NSW Parliament in 1995 and served in the Legislative Council until his retirement in 2015. He was appointed Parliamentary Secretary for Veterans Affairs under Premiers Barry O'Farrell and Mike Baird from 2011 - 2015. In 2018 he was awarded an OAM for his services to the people of NSW and the NSW Parliament.

He is a Trek Leader for Adventure Kokoda and a developer of the Kokoda Youth Leadership Challenge. He held the New South Wales 24-hour Ultra Marathon record in 1985 and 1986.

Lynn was instrumental in establishing the Kokoda Track Authority with the Government of Papua New Guinea in 2004 to ensure local villages along the Kokoda Trail received an equal share of benefits from the emerging trekking industry. He has lobbied the Australian Government to produce a Master Memorial Plan to protect, honour and interpret Australia and Papua New Guinea's shared wartime heritage across the Kokoda Trail.

In 2015 the Government of Papua New Guinea inducted Lynn as an Officer of Logohu in their New Year's Day Honours and Awards list for 'for service to the bilateral relations between Papua New Guinea and Australia and especially in the development of the Kokoda Trail and its honoured place in the history of both nations' over the past 25 years.

Lynn was appointed to the Board of the Kokoda Track Memorial Walkway at Concord. He was elected President of the NSW Parliamentary Lions Club and was a Trustee on the Anzac Memorial building in Sydney. He was Chairman of the Campbelltown Chamber of Commerce and Industry between 1993 and 1994.